Ammonia borane, also referred to as ammonium borane and borazane, has been investigated as an energy-dense source of hydrogen, for example, for use in hydrogen related power generation. Methods of synthesis of ammonia borane are disclosed in Ramachandran et al. US 2007/0243122 A1 (“Ramachandran”). Briefly, the process involves reacting a metal borohydride with an ammonia salt, in a suitable solvent. For example, sodium borohydride is reacted with ammonium carbonate in an ether solvent, such as tetrahydrofuran (“THF”) or dioxane. Following the reaction to form ammonia borane, the solution is filtered and the solvent is removed under vacuum to yield solid ammonia borane. The recovered ammonia borane powder may be purified by extraction with a suitable solvent, followed by removal of the solvent under reduced pressure, to yield a solid ammonia borane of relatively high purity.
A shortcoming of prior art ammonia borane purification methods is that many of the impurities present in the crude ammonia borane are also soluble in the solvent used to extract the ammonia borane. Consequently, when the solvent is evaporated from the ammonia borane solution, such impurities remain in the recovered product. Thus, despite the methods disclosed by Ramachandran and others, a need remains for an ammonia borane purification method that is economical to operate commercially and yields high-purity ammonia borane.
It can be understood that metal ammonium salts, such as sodium ammonium carbonate, are a significant byproduct of the synthesis reaction between a metal borohydride and an ammonia salt. Isolating ammonia borane from metal ammonium salts and other byproducts of the reaction has proven difficult in the past, due to the difficulty of filtering byproducts that have precipitated in the reaction slurry and due to the similar solubilities of ammonia borane and the byproducts in a range of polar and non-polar solvents. The utility of ammonia borane as a hydrogen storage medium is linked to its ability to release H2 over a wide range conditions, including in the presence of ionic liquids and bases. Accordingly, purification of crude ammonia borane is limited to conditions that do not trigger decomposition of the ammonia borane and H2 release.